1. Field of the Disclosure
Embodiments disclosed herein generally relate to an apparatus and method for use, and more specifically to valves and valve control apparatus and methods for controlling flow in oil and gas wells.
2. Background Art
In a typical oil and gas well a casing string extends from the ground surface to a ground formation containing hydrocarbons to be produced. A production tubing string extends within the casing string. A packer is provided downhole to prevent flow of hydrocarbon and other fluids in the annulus between the production tubing string and the casing string. A portion of the casing extends below the packer and the bottom of the production tubing string. Fluid flow is typically from the formation through perforations in the portion of the casing below the packer, into the production string, and through the production tubing string to the wellhead. Fluids may be liquid or gaseous state and may include, among other things, oil, gas and water.
Surface control subsurface safety valves (“SCSSV”) are used to prevent uncontrolled flow of reservoir fluids through the tubing string. A SCSSV is threadably inserted within the production tubing string and forms a portion of the production tubing string after insertion.
Typically, the SCSSV is positioned at least several hundred feet below the surface of the earth, or in a typical offshore well, several hundred feet below the mudline. A typical SCSSV comprises a single flapper valve that is rotatably disposed within the valve, and rotates downward to an open position, and is fail-safe rotated upwardly by a spring, and thereby closed. Usually, the SCSSV comprises an internal sliding cylindrical sleeve or flow tube that is spring biased in an upward direction. When the flow tube is in its uppermost position, it permits the flapper to rotate (under spring bias) to its closed position, blocking the bore of the flow tube and consequently closing the valve. When the flow tube is forced downward against its spring bias and the spring bias of the flapper (the flow tube maybe moved by hydraulic or electric means, as is known in the art), the flow tube forces the flapper out of the bore of the valve, thereby opening the valve. Reversal of this procedure moves the flow tube upward, out of the way of the flapper, which then rotates back to its closed position. It can be appreciated that any upward fluid flow during this closing process tends to force the flapper into its closed position.
As mentioned above, flow tubes within SCSSV may be controlled hydraulically or electrically from the surface. For hydraulically operated valves, hydraulic pressure is applied down a control line to a fluid chamber in the SCSSV, forcing the flow tube to slide downwards thereby pushing the flapper downwards to open the valve. When hydraulic pressure is removed, the fail-safe closure spring on the flow tube pushes the flow tube back up and exposes the flapper to the flowing fluid, causing the flapper to shut. The flapper will fail-safe shut with no flowing fluid when the sleeve is removed.
Flapper valves are susceptible to failure for various reasons including corrosion, sand, slam closures, debris, wireline cuts, and other downhole operating conditions. In prior art SCSSVs, upon failure of the flapper valve, installation of a secondary valve within the SCSSV may be required. A typical installation of the secondary valve within the SCSSV requires two wireline processes. The first process involves using a wireline tool to lock out the flapper within the SCSSV. The second process involves using a wireline tool to install a secondary valve within the SCSSV.
U.S. Pat. No. 5,293,943 to Williamson, Jr. teaches a downhole, inline well safety shutoff valve having a spring-loaded, normally closed flapper shutoff flapper element that may be opened by a downwardly driven movement of an operator tube coaxially and slidably disposed within the tubing string bore. A rod structure drives the operator tube.
U.S. Pat. No. 7,178,600 to Luke, et al. teaches a downhole deployment valve including fail safe features such as secondary valve members, an upward opening flapper valve or a metering flapper below a sealing valve.
U.S. patent application Ser. No. 11/041,393, Publication No. 2006/0162939 by Vick, Jr. et al. describes a valve system for use in a subterranean well, the valve having multiple closure devices.
It can be appreciated that it would be desirable to have a SCSSV assembly which comprises multiple flapper valves, with only one flapper valve in service at a time, so that in the event the flapper valve in service becomes comprised (i.e., leaks) then another flapper valve can be put in service.